Flocculation of Metal Hydroxides with Polymers - Optimization and Kinetic Modelling

Abstract

<p>This study determined optimum conditions for the use of polymers in flocculation of metal hydroxides present in neutralized mine drainage. Using the criterion of supernatant particulate metal concentration following flocculation and settling, optimum polymer properties and mixing conditions were obtained for both strong and weak simulated minewaters containing iron. It was shown that these conditions also provided minimum metal residuals (less than 0.3 mg/l) for simulated minewaters containing copper or zinc. For two-metal systems, the residual particulate metal concentrations achieved were lower than for the corresponding single-metal systems.</p> <p>Near the optimum, mixing conditions were more important than polymer properties. Optimum mixing times decreased greatly as minewater strength increased, as would be expected from kinetic considerations. Optimum mixing speeds increased moderately with increasing initial metal concentration, probably because of a corresponding increase in floc strength. Polymer molecular weight had no effect over the range investigated. The polymer degree of hydrolysis was unimportant in the range from 2 to 35 percent for the three metals at various initial concentrations. The optimum polymer dosage was not narrowly defined, but could be related to the minewater strength as 1.7 x 10‾³ the initial metal concentration.</p> <p>Experiments performed to determine the best kinetic model for the process demonstrated that it could not be represented by a simple aggregation model and that none of the available models incorporating floc breakup were adequate. A new model was proposed which incorporated for the first time the decrease in the aggregation rate with time because of the shortening of the absorbed polymer loops, and the existence of a critical mixing intensity for floc breakup. This model, which is second order in particle concentration, was found to predict satisfactorily the results obtained with the simulated iron, copper and zinc minewaters. Although the aggregation rate was greater for zinc than for iron or copper, the achievable supernatant particulate concentration was indepedent of metal type. The critical mixing intensity for floc breakup was found to increase with increasing initial metal concentration.</p> <p>The model was tested on three actual minewaters and was found to give satisfactory predictions.</p>